Diffusion DepositionDiffusion DepositionDiffusion Deposition

• At a given temperature, the vapor pressure over a water surface is greater than that over an ice surface.

• Due to this gradient, water vapor moves from the higher pressure surrounding the droplets to the lower pressure surrounding the crystals.

• Thus, the ice crystals grow at the droplets' expense. This process creates sub-saturation with respect to water, and the droplets evaporate to maintain water saturation, making additional water vapor available for ice crystal growth.

• Eventually the pool of liquid water diminishes and the cloud becomes glaciated.

Diffusion Deposition (cont.)

• The rate of diffusion is highest where the difference between the saturation vapor pressure between ice and supercooled liquid water is greatest. Optimal growth rates occur near -15°C.

Summary Layer 1 (Ice Producing): -12C for ice. Layer 2 (Warm Layer?)

Warm Layer Maximum Precipitation Type Precipitation Type

Temperature with ice introduced without ice introduced

<0.5C Snow Freezing Rain/Drizzle

0.5C to 3C Mix (1C) to Sleet (3C) Freezing Rain/Drizzle

> 3C Freezing Rain/Drizzle Freezing Rain/Drizzle

Evaporation or Sublimation Layer? Layer 3 (Surface) ● 0C at surface or lower? ● Can FZRA turn to IP (trend to colder/deeper air)? SN? -12C? ● Tw > 33F possibly rain (greater than 1000ft)